Idea: Large-diameter plastic support frame surrounded by thin layer of copper
To ensure maximum current flow through the loop conductor, ideally a large-diameter copper tube would be used for the loop conductor. For this project I wanted a conductor diameter of 10 cm. However, 10-cm-diameter copper tube is not available for me to purchase locally, and even if it were, I lack the equipment needed to form a loop conductor from such a thick copper tube; this would require either bending the 10-cm-diameter copper tube into a loop, or soldering several short segments together to form a loop-like shape. Either bending or soldering such a huge-diameter copper tube would require specialised equipment.
As an alternative approach, I decided to construct a plastic support frame consisting of 10-cm-diameter plastic tubes. The surface of this support frame will then be covered by a thin layer of copper sheet metal, to form a large-diameter conductive copper tube.
A later article will show how to apply the conductive copper sheet on top of the plastic support frame.
Square cross section instead of circular cross section
The 10-cm-diameter plastic tubes were constructed manually (see next section), because no suitably light and strong 10-cm-diameter plastic tubes could be found.
For ease of construction, the 10-cm-diameter plastic tubes have a square cross section instead of a round cross section. This will make easier the subsequent application of the conductive copper sheet, as the copper sheet can be affixed to the flat tube surface, which is easier than attempting to affix a copper sheet to a round tube surface.
Furthermore, the RF resistance of a square-cross-section tube (having a side length of r centimeters) is the same as the RF resistance of an equivalent-diameter round-cross-section tube having radius of r centimeters. This result was published by H. A. Wheeler in 1955, in an article titled "Skin Resistance of a Transmission-Line Conductor of Polygon Cross Section". A quote from Wheeler's abstract follows.
If a conductor cross section is any straight-sided polygon that can be circumscribed on a circle, it is found to have the same skin resistance as a conductor whose cross section is this circle. For example, a square wire has the same resistance as a round wire of the same radius, though the square perimeter is 4/pi times as great.
An equivalent result can be found by using W9CF's formulas for computing the approximate RF resistance of rectangular cross section conductors.
Finally, and most reliably, the equivalent result can be verified by finite-element method (FEM) electromagnetic field simulation software, which allows not only computation of the RF resistance of a square-cross-section conductor, but also allows computation and visualisation of the actual RF current distribution within the conductor. A future article will describe a FEM simulation showing the equivalent RF resistance of a square-cross-section conductor and an equivalent-diameter round-cross-section conductor.
As a sneak preview of the upcoming FEM analysis, below are some screenshots of my previously-conducted analysis of current density in a square-cross-section conductor consisting of four separate flat copper sheets. The simulation results need to be checked for plausibility and checked against analytical predictions, but current results initially seem quite plausible.
Constructing the plastic tubes
A significant challenge was finding tube material that was sufficiently lightweight to allow easy cutting and joining with hand tools, but at the same time was sufficiently strong to support itself and the weight of a vacuum variable capacitor at the top of the loop.
Ordinary 10-cm-diameter PVC pipe was ruled out for two reasons: (1) it is too heavy and unwieldy for this project and, more importantly, (2) its round cross section would make surface application of copper flashing quite difficult.
I settled upon using corrugated plastic sheets that were available from my local hardware store in dimensions of 91 cm x 45 cm. The thickness of the corrugated sheet is about 2 mm. This construction is extremely lightweight, while the corrugation provides some degree of structural strength.
The plastic sheet was lightly scored, at 10-cm intervals, with the blunt tip of a pen.
The sheet was then folded along the scored lines, to form a square-cross-section tube.
Next the placement of the vacuum variable capacitor was investigated. Below, the vacuum variable capacitor is placed inline with the top tube. This arrangement would probably result in the lowest RF resistance (allowing the current to flow directly from the tube to the variable capacitor with minimal change of direction).
However, the above inline method of capacitor mounting is challenging to construct because it provides no easy way of removing the variable capacitor, requiring the loop to be constructed bit by bit around the variable capacitor, eventually resulting in a solid structure encasing the capacitor. This was deemed too difficult for this project. Nevertheless, an example of a successful construction of this type can be seen in JL1BOH's loop, at http://www.aa5tb.com/jl1boh_04.jpg, which is explained at the bottom of AA5TB's page http://www.aa5tb.com/loop.html. Another successful construction of this type, using inline horizontal mounting of the vacuum variable capacitor, can be seen in IZ2JGC's loop antenna here: https://plus.google.com/u/0/117232231941836334584/posts/D2v9Ht7yzNF.
Below, the traditional vertical orientation of the vacuum variable capacitor is attempted. This could result in a slight increase in RF resistance as all the RF current from the tube is forced to flow in the small and thin straps that connect to the variable capacitor. One representative strap is shown below by the yellow strip of paper. In addition to the strap resistance, the RF current is also forced to change direction from horizontal flow in the tubes, to vertical flow through the capacitor. Possibly, a future article may investigate numerical simulations and visualisation of the RF current density when the current is forced to flow from the wide-diameter tube into the small, thin strap. In any event, for mechanical simplicity, likely the variable capacitor will/must be mounted in the vertical position for this project.
Below, the traditional vertical orientation of the vacuum variable capacitor is attempted. This could result in a slight increase in RF resistance as all the RF current from the tube is forced to flow in the small and thin straps that connect to the variable capacitor. One representative strap is shown below by the yellow strip of paper. In addition to the strap resistance, the RF current is also forced to change direction from horizontal flow in the tubes, to vertical flow through the capacitor. Possibly, a future article may investigate numerical simulations and visualisation of the RF current density when the current is forced to flow from the wide-diameter tube into the small, thin strap. In any event, for mechanical simplicity, likely the variable capacitor will/must be mounted in the vertical position for this project.
Next, the remaining plastic sheet was cut into corner-shaped pieces to join the tubes together. In the below image, the yellow paper indicates the size of one corner piece.
The pattern for two corner pieces looks as follows.
Cutting and separating the corner pieces is shown below. Note the uneven edges along the connecting seam still need to be trimmed to be straight.
The final collection of 8 corner pieces is shown below.
Affixing the straight tube segments to the corner pieces with double-sided tape gives a loop-like shape.
The loop is strong enough to support itself, though slight sag is evident at the top due to the gap in the loop.
Next steps
The structural strength at the corners should be improved, as currently the corner supports only consist of two flat L-shaped sheets.
The current taped-together construction should be redone with glue. An appropriate glue must be investigated.
It must be investigated how to support the heavy vacuum variable capacitor at the top gap of the loop. Possibly, a separate tripod-like stand and vertical pole (spine) could support the heavy capacitor independently. Then, some sort of arms extending from the spine could support the loop. The loop would then be laid or "hung" on top of the arms extending from the spine. Alternatively, some vertical or diagonal support braces could be introduced on each side of the gap to prevent sagging. It must however be kept in mind that the structural strength of the corrugated plastic -- even with additional bracing -- might not be great enough to withstand the stress caused by the weight of the vacuum variable capacitor. The corrugated plastic walls of the tubes (that would need to bear the force of the bracing) could buckle or collapse if subjected to too much weight. This points to a separate spine as the likely solution, so that the spine would bear the weight of the capacitor and the loop, thus freeing the fragile loop from bearing any weight, and only requiring the thin-walled loop to be sturdy enough to avoid collapsing on itself -- which it already is.
Once the loop structure and supporting structure have been finalised, the next big step will be applying an electrically-continuous copper sheet to each of the four flat faces of the loop surface.
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